426 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, No. 19 
in the core the density decrease with depth. On the other hand the 
value 3.25 leads to an unreasonably high density at the center. For 
this reason the density at the top of the basic layer has been taken 
as 3.35, corresponding, as stated above, to a normal density 3.3 and 
to a density 4.35 at 1600 km. The density of the iron would then be 
9.5 at 3000 km and 10.7 at the center. 
As a tentative distribution and as a basis for future speculation 
let us therefore suggest: (1) an outer layer 60 km (about 35 miles) 
thick in which the material changes more or less gradually from gra- 
nitic to something more basic than a gabbro; (2) a shell extending to 
Fig. 4. Diagram intended to suggest the segregation of metallic iron toward the 
center, and the zone of pallasite (mixture of iron and silicates) surrounding the central 
core. 
a depth of 1600 km, consisting of peridotite, that is, mainly of iron- 
magnesium silicates and having a normal density 3.3 and a density 
at 1600 km of 4.35; (3) a shell of pallasite reaching to 3000 km below 
the surface, in which silicate rock is gradually replaced by metallic 
iron (or nickel-iron) not yet completely segregated, the density in 
this shell changing gradually from 4.35 to 9.5; and (4) below this 
layer of pallasite a central core of nickel-iron of nearly constant 
density—varying from a little below to a little above 10. The exist- 
ence of other layers or of other discontinuities is neither affirmed nor 
